In a scanning electron microscope (SEM) a focussed electron beam is rastered over the surface of a specimen. At each point on the raster, an electron detector is used to record the varying signal due to electrons that have been scattered or emitted from the specimen and thus form an image of the rastered field of view. When a small object or feature, such as a particle or an inclusion, is present on the surface of a specimen and this feature is to be analysed, the focussed electron beam is directed on to the feature and X-rays are emitted. An X-ray spectrum can be collected with a suitable detector and used to identify the chemical elements present in the feature. Different types of electron detector can be used to record either the low energy secondary electrons, where the signal is affected strongly by specimen topography, or the high energy scattered electrons, where the signal is affected strongly by the mean atomic number of the material. However, none of these signals provides any information on the crystal structure if the feature happens to contain crystalline material.
Many compounds that have the same chemical composition can have different crystal structures and behave differently (e.g. different forms of TiO2). Furthermore, the crystalline structure can suggest under what conditions (e.g. temperature and pressure) the material was formed which can be useful in forensic investigations. If the feature is crystalline, some electrons and X-rays travelling in very specific directions will be reflected from crystalline planes. Because SEM images are built up in serial fashion by raster scanning of the incident beam, X-ray and electron detectors used in SEMS are typically non-imaging. However, with a suitable camera, typically fitted with a phosphor, it is possible to collect an image of a pattern due to electron or X-ray diffraction (Kikuchi or Kossel) provided the camera has direct line of sight of the feature. The pattern corresponds to the material under the focussed electron beam present at any time.
An electron backscatter diffraction (EBSD) pattern may be obtained using a geometry that is typically as shown in FIG. 1 where the pattern is formed from electrons scattered from just beneath the specimen surface that are Bragg-diffracted by crystalline layers within tens of nanometres of the specimen surface. The high tilt angle of the sample improves pattern contrast. In FIG. 1, EDS is the X-ray detector. EBSD is the camera for collecting an electron diffraction pattern that is formed by electrons striking the phosphor screen. BSED and FSED are solid state electron detectors positioned to detect electrons scattered backwards or forwards (respectively) relative to the point of impact of the incident focussed electron beam which is travelling downwards and striking the sample at an angle of 20 degrees. This angle is commonly taught but other angles can be used.
The accuracy of crystallographic parameters determined by EBSD may not be sufficient to identify a material uniquely and, as explained in U.S. Pat. No. 6,326,619, determination of the chemical elements by X-ray analysis can assist in crystal phase identification. Although the spatial resolution of EBSD is typically better than 100 nm, the X-ray signal is generated over a much larger volume (typically 1000 nm or more for a 20 keV incident electron beam). Consequently, if information is required on features much smaller than 1000 nm, the information from EBSD and X-ray is not complementary because the X-ray spectrum may contain X-rays from elements that are not in the feature but are in nearby peripheral regions.
For EBSD, the contrast of the sharp lines caused by Bragg reflection from crystalline planes against the background of diffusely scattered electrons is improved if the surface has a high tilt with respect to the incident beam. However, this high tilt increases scattering in the direction of tilt and results in spatial resolution that is typically 3 times worse down the tilted surface compared to parallel to the tilt axis. For irregular objects, there can be problems orienting the microscope stage to get a surface of the feature of interest into the line of sight of the X-ray detector, or the camera with a suitable orientation to collect EBSD. If the surface is rough or faceted or has an amorphous superficial layer, it may not be possible to obtain a diffraction pattern. Therefore, it is particularly difficult to obtain EBSD patterns from unprepared irregular specimens such as forensic samples or samples for failure analysis.
Therefore, there is a need for a method to analyse the crystalline structure of a small feature with a spatial resolution much better than 1000 nm and preferably better than 100 nm. This need is particularly strong when analysing irregular specimens. Furthermore, for routine application in forensic or failure analysis, the method needs to be efficient.